1. Field of the Invention
The present invention relates to lasers. More specifically, the present invention relates to solid-state slab lasers having high beam quality along orthogonal transverse axes.
2. Description of the Related Art
Solid-state lasers are useful in a number of military and commercial applications. Many applications require high output power levels and high beam quality in order to achieve adequate beam intensity on a target in the far field. Some of these applications also require short duration pulses, which may fall into the nanosecond range. Military applications include rangefinders, designators, active tracking illuminators, beacons for atmospheric wavefront sensing, laser radar, infrared countermeasures, and directed energy weapons. Commercial applications include industrial materials processing, long-range remote sensing, and other scientific applications. Solid state lasers employ a doped insulator lasing medium that can be configured into shapes such as cylindrical rods and rectangular solids. Cylindrical rod lasers are particularly useful, but have limited scalability for a variety of reasons, including thermal management limitations.
One approach to scaling solid-state lasers to higher power levels involves using a high aspect ratio rectangular slab lasing medium geometry. The high aspect ratio slab geometry provides a larger cooling surface relative to a cylindrical rod of equal volume. Further, the heat flow within the slab is essentially one-dimensional and is spread over a large area; therefore the temperature gradient is small relative to the rod and is also one-dimensional. Because the stress within the slab follows the temperature gradient, stress-induced birefingence tends to be along the normal to the broad slab surfaces and light polarized in this direction, or an orthogonal direction, will not be depolarized when propagated through the slab. The thermal lensing effect present in a slab medium is cylindrical and can be compensated by propagating the beam in a zigzag path between the broad slab surfaces.
Conventional high aspect ratio slab laser resonators typically exhibit different beam quality in the two transverse dimensions that are oriented along directions corresponding with the wide and narrow axes of the slab. This is because the slab laser operates with a different resonator configuration along the wide axis, as compared to the narrow axis of the slab. A simple stable laser resonator system built around a high aspect ratio slab lasing medium can be easily designed so that the narrow axis produces single transverse electromagnetic (TEM) mode performance. However, the wider axis typically produces multi-mode beam quality unless a more complex resonator design is used in this axis. Accordingly, for typical stable resonators, a low Fresnel number is present along the narrow, single mode, axis and a larger Fresnel number for the wide slab axis. The laser therefore produces poor beam quality along the wide axis of the slab.
Low-order transverse mode operation along the wider slab axis has been achieved in the prior art. This is accomplished in a stable laser resonator by providing a limiting aperture sized to achieve a low Fresnel number within the system. Intra-cavity telescopes are sometimes used in laser resonators to demagnify the limiting aperture and reduce the effective Fresnel number. Because a high aspect ratio laser slab is broader in one transverse dimension, achieving low Fresnel number operation along the wide direction requires a physically long resonator length. This approach has two fundamental disadvantages. First, the long length of the resonator results in a large physical volume for the laser resonator. Second, the buildup time in a long resonator is slower, leading to long duration Q-switched pulses. Furthermore, because the transverse mode selection for the two orthogonal directions is decoupled, the resonator may produce a beam with a different beam quality in each direction, which is undesirable in certain applications.
A low order mode in the broad dimension of the slab can also be achieved using an unstable resonator configuration, in which the out-coupling is achieved, for example, by varying the reflectivity of an out-coupler mirror across the transverse dimension. Other structure in the lasing cavity can be used to correct beam quality as well. A gaussian reflector can be used to reduce higher order TEM modes of the beam. The unstable resonator approach cannot be used with cavity dumping, and thereby limits its utility for certain pulsed applications of interest, such as short-duration pulsed systems.
Thus, there is a need in the art for a laser resonator that produces uniformly high beam quality in both transverse dimensions with a compact high aspect slab lasing medium that also addresses the problem of generating short-duration Q-switched pulses in a low-Fresnel number resonator, where the length of the resonator is determined by mode discrimination associated with the narrow dimension of the high-aspect slab.